Replacing a circuit breaker in MCB

I killed a 30A circuit breaker (MK LN5930) in my MK Sentry consumer unit. Don't ask how - it was silly! It is dedicated to an oven which is in need of repair and now trips the main switch in the consumer unit.

I have found a replacement MCB and wondering is it easy to replace these?

Its quite easy. Just be extremely cautious. The main incoming cables to the consumer unit are always live - unless someones fitted an upstream isolator.

Turn off mainswitch (and isolate elsewhere if you can).

Test for dead - go round, turn lights on, a radio etc.

Use your multimeter to test the busbar (via the mcb bottom screws if accessible) for dead. To be sure, turn the lectric back on and retest for live (then you know your multimeter and lights are working) - then back off and retest for dead.

Now, unscrew the outgoing live wire from the MCB. Then unscrew the bottom screw, which holds the busbar prong. Now, at the back of the MCB is a clip that holds it onto the DIN rail. At the bottom will be a slot to insert a screwdriver and prise downwards. This will release the clip and you can lift the MCB out.

You may find it a little easier to loosen the busbar from the other MCBs too, since this can hold the MCB in place on its own. I found it easier doing it this way when I changed my Hager MCBs. MK look a similar design.

Steve gives generally good advice, but I would be very wary of using a second hand mcb and relying on it to still work as promised. (There is no easy way to check an MCB trips at the stated rating).

When you say the oven now trips the 'main switch' in the CU, I presume you mean you have an RCD incomer? Any chance of a picture?

Legally, I would say this counts as replacing a damaged component so would not be notifiable under Part P. However, with an older install like yours , an hours worth of a local electrician's time to replace the mcb and at the same time cast an eye over the whole installation may well be money well spent.

The same situation faces everyone who buys a house and doesn't replace the CU with a new one...

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True, but the mcbs are more likely to have come new from the manufacturer and not in a jiffy bag via the Royal Mail's careful handling...

And of course the original certificate and 10 yearly PIR documentation will show you the history.... (When was the last time someone turned up to an install older than 3 years and actually found an installation certificate!)

Actually, I have no idea what the MCB failure rate is (I know the RCD rate is rather higher than one might think) and I've never seen an instrument designed for testing trip times/ratings, but can't immediately think of a reason why one couldn't be developed for onsite use?

I'm certainly not saying the OP shouldn't replace the MCB, was just sounding a note of caution.

Actually, I have no idea what the MCB failure rate is (I know the RCD rate is rather higher than one might think) and I've never seen an instrument designed for testing trip times/ratings, but can't immediately think of a reason why one couldn't be developed for onsite use?

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The problem is the currents involved. RCDs are tested with relatively small currents so they are easy and safe to test.

MCBs are a whole different ball game. The current needed to test fast trip on a MCB would range from 30A for a B6 to 1260A for a D63 . Such currents could not be safely passed through probes and for the larger breakers could also cause issues on the supply side. And if they are faulty such high currents may cause violent failure.

IMO the only safe way to test a MCB would be to remove it from the board and place it in a suitable enclosure to resist violent failure, then feed the current to it from a special power supply pack. The power supply pack would need to be able to supply significant voltage as the current dropped off to test the MCBs ability to safely break the arc.

The power supply pack would need to be able to supply significant voltage as the current dropped off to test the MCBs ability to safely break the arc.

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There are probably some things you'd have to take as read, and that would be one. For a manufacturer to test samples is one thing, but you'd actually want to avoid arcing in something you were going to put back.